1. Field of the Invention
This invention relates to a light-emitting device and a method of manufacturing the light-emitting device.
2. Description of the Related Art
The conventional light-emitting device is constructed such that it comprises a light-emitting element such as an LED (light-emitting diode) which is mounted on the bottom of U-shaped cup formed in a portion of a supporting body such as a packaging substrate, and a wavelength conversion member placed in the cup so as to cover the light-emitting element, the wavelength conversion member comprising a light-transmitting material containing a fluorescent substance (phosphor). This wavelength conversion member is manufactured by a process wherein a fluorescent substance is incorporated, at first, in a material for a light-transmitting member such as silicone resin or glass, and the resultant material is then dripped, by using a dispenser, into a recess where a light-emitting element is mounted, the material thus placed in the recess being subsequently thermally cured.
Since the specific gravity of fluorescent substance is several times as high as that of the liquid resin which is not yet thermally cured, most of the fluorescent substance in the resin is caused to densely flocculate and sediment around the LED chip. Although the fluorescent substance located in the vicinity of the surface of the LED chip is capable of effectively absorb the light from the LED chip, most of the fluorescent substance cannot absorb the light from the LED chip but shield the light on the contrary, thus attenuating the energy of light. As a result, the light-emitting output of the light-emitting diode deteriorates. Although it is possible to inhibit the sedimentation of fluorescent substance through the employment of a fluorescent substance of small particle diameter, the light extraction efficiency and light absorption efficiency thereof deteriorates as compared with a fluorescent substance of large particle diameter. Further, if a fluorescent substance of small particle diameter flocculates, the mechanism of scattering the light thereof would increase.
Generally, as the particle diameter of fluorescent substance becomes larger, the light conversion efficiency thereof becomes more excellent. However, since the fluorescent substance becomes more liable to sediment as the particle diameter thereof becomes larger, it is difficult to form a layer of fluorescent substance where the fluorescent substance is uniformly dispersed.
Further, there has been also proposed a light-emitting element where the fluorescent substance is deliberately sedimented. Even in this light-emitting element having such a structure, the fluorescent substance which fails to contribute to the emission of light exists to a great extent. Further, since the emission of light is shielded by this fluorescent substance, the optical output deteriorates. Further, since the distribution in density of fluorescent substance differs depending on the configuration of LED chip and packaging cup as well as the location of wiring, the problem of irregular coloration generates. Unless it is possible to control the dispersibility of fluorescent substance, the optical characteristics such as chromaticity and optical output become non-uniform depending on individual light-emitting device, thus deteriorating the yield in the manufacture of light-emitting device.
Furthermore, since a lot of resin is required to be employed in the conventional coated structure of fluorescent substance, air bubbles are liable to be generated in the package cup, thus giving rise to the generation of irregular coloration or disconnection of wire due to cracking. No one has succeeded as yet to find out a technique to coat a fluorescent substance of large particle diameter with a uniform density distribution.